This invention relates to an antilock control system for vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters which include the road surface conditions and the amount of slip between the wheel and the road surface. The braking force increases as slip increases, until a critical value of slip is surpassed. Beyond this critical slip value, the braking force decreases and the wheel rapidly approaches lockup. If the wheel is allowed to lock, unstable braking occurs, and vehicle stopping distance on nondeformable surfaces increases. Thus, stable vehicle braking occurs when wheel slip does not exceed this critical slip value. An antilock control system achieves stable braking and minimizes stopping distance by cycling brake pressure such that braking force is maximized. This is accomplished by first detecting an incipient wheel lock condition, which indicates braking force has peaked and is now decreasing. One criteria that is used to indicate incipient wheel lock is excessive wheel deceleration and/or excessive wheel slip. Once an incipient wheel lock has been detected, pressure is relieved at the wheel brake. Upon releasing the brake pressure, the wheel accelerates and begins approaching recovery. The wheel is later said to be recovered when wheel slip is reduced to a value below the critical slip value. When the wheel has substantially recovered, brake pressure is reapplied. Reapplication of brake pressure results in the wheel again approaching lockup and the wheel cycle process is repeated.
In terms of the critical parameters of wheel slip and acceleration, the wheel cycle can be better described as follows: (a) the wheel is forced to decelerate and wheel slip increases until (b) the critical slip value is surpassed, at which point (c) an incipient lock condition is detected. Upon detecting the incipient lock, the wheel is allowed to begin reaccelerating. As (d) the wheel continues to accelerate, slip decreases, until (e) the wheel slip falls below the critical slip value. The wheel having substantially recovered, it is then forced to decelerate again, and the wheel cycle process repeats, returning to step (a). To achieve this wheel cycle, wheel brake pressure is increased during steps (a) and (b), is released during step (c), and is increased again after step (e). During step (d), when the wheel is accelerating and wheel slip is decreasing, it is not always necessary to continue relieving pressure as in step (c), yet the wheel is still too unstable to proceed to increase pressure as is done after step (e). This is due to the fact that, although the wheel is accelerating and approaching recovery, wheel slip is still above the critical slip value. Thus the wheel is operating in the region characterized by unstable braking. But the wheel is also approaching stability because wheel slip is decreasing. Therefore, in this region where the wheel is still operating in the unstable region and is approaching recovery (i.e. approaching stability), it is desirable to maintain pressure at a value that, while allowing the wheel to continue recovering, will avoid unnecessary pressure decrease. This state is called a "hold". The complete wheel pressure cycle can now be described as follows: wheel brake pressure increases, which (a) forces the wheel to decelerate and increases wheel slip until (b) the critical slip value is surpassed, at which point (c) an incipient lock condition is detected. Upon detecting the incipient lock, the wheel brake pressure is released, which allows the wheel to begin reaccelerating. As (d) the wheel continues to accelerate, slip decreases, at which point pressure is held constant to prevent unnecessary pressure loss. Once (e) the wheel slip falls below the critical slip value, the wheel has substantially recovered. Pressure is reapplied, the wheel is forced to decelerate again, and the wheel cycle process repeats, returning to step (a).
It should also be briefly noted that braking efficiency is maximized when the amount of time spent braking while wheel slip is at or near the critical slip value is maximized. This means that pressure should be released only enough to allow the wheel to return to the stable braking region, and, once operating in the stable region, pressure should be reapplied to a significant fraction of the pressure required to produce critical wheel slip. In doing so, the amount of time spent at or near the critical wheel slip is maximized.
To achieve linearity of control, antilock control systems have been known to utilize a motor driven piston actuator as opposed to a solenoid modulated actuator. Both mechanizations allow for the application, relief and holding of pressure, but the motor driven actuator has several advantages. First, the motor driven system controls pressure in a continuous rather than discrete fashion. Secondly, the motor driven mechanization, because of the relationship between motor torque and piston head pressure, provides information regarding wheel brake pressure without the addition of an external device such as a pressure transducer. Thus, while most antilock control systems execute wheel cycle control based upon vehicle motion parameters such as wheel slip and acceleration, the motor driven systems are automatically afforded the additional parameter of measured brake pressure.
However, the inertial characteristics of a motor driven actuator create an antilock control system which is under-damped. These inertial characteristics can be better understood by realizing the following dynamic relationships: (a) when the motor load (i.e. the pressure present on the piston head) is equal to the motor torque, the motor does not rotate and the piston remains stationary. Conversely, (b) when the motor load is small compared to the motor torque, the motor rotates at a high rate and the piston travels at a high speed. Because the objective of antilock control is to cycle the brake pressure closely about the pressure required to produce critical wheel slip, (c) optimal control is achieved when the piston moves slowly, which increases pressure gradually. Thus, (d) if the motor torque is significantly greater than the pressure present at the piston head (i.e. the motor load), the piston moves too rapidly to produce the desired pressure control, the system becomes over-excited and overshoots the desired pressure. Therefore, care must be taken in the control of a motor driven actuator to avoid situations in which the actual wheel brake pressure is substantially lower than the desired wheel brake pressure.
As a by-product of controlling the antilock control system such that inertial overshoot is avoided, several additional advantages can be realized. First, by avoiding the over-release situation, braking efficiency is increased. This result is two-fold: (1) because minimum brake pressure is maintained at a level higher than would be realized if an over-release condition was in existence, and (2) because brake pressure does not subsequently overshoot the desired pressure, the minimum brake force does not vary as greatly from the optimum brake force for the surface. Secondly, by avoiding unnecessary pressure release, the amount of time spent by the system in reaching the optimum brake pressure for the road surface is minimized. These advantages result in higher braking efficiency. Furthermore, when braking efficiency is maximized, the "vehicle surging" effect felt by the driver is minimized. This surging sensation is created by large changes in vehicle deceleration. By maintaining braking efficiency, the deceleration of the vehicle is more consistent, thereby creating a more pleasant vehicle braking sensation for the driver.